A twin-resonance-coupling phenomenon and the sensing characteristics of a selectively fluid-filled microstructured optical fiber (SFMOF) are proposed and demonstrated. The SFMOF is realized by selectively infiltrating refractive index fluid into a single air hole located at the second ring near the core of the MOF. Twin-resonance dips are observed in the transmission spectrum. Theoretical and experimental investigations reveal that the twin-resonance dips both result from the coupling between LP(01)(C) silica core mode and LP(01)(L) liquid rod mode. Their sensitivities strongly depend on the dispersion curves of the silica and fluid material. Sensitivities of 290 nm/°C (739,796 nm/RIU) and 591.84 nm/N (701.2 pm/µɛ) are achieved, which are the highest for a SFMOF-based device to date, to our best knowledge. Furthermore, the twin-resonance dips appear to shift in the opposite directions with changes in temperature or axial strain, providing a method to achieve two- or multi-parameter measurement in such a compact structure.
A tunable microstructure optical fiber for different orbital angular momentum states generation is proposed and investigated by simulation. The microstructure optical fiber is composed of a high refractive index ring and a hollow core surrounded by four small air holes. The background material of the microstructure fiber is pure silica. The hollow core and the surrounded four small air holes are infiltrated by optical functional material whose refractive index can be modulated via physical parameters, leading to the conversion between circular polarized fundamental mode and different orbital angular momentum states at tunable operating wavelengths. A theoretical model is established and the coupling mechanism is systematically analyzed and investigated based on coupled mode theory. The fiber length can be designed specifically to reach the maximum coupling efficiency for every OAM mode respectively, and can also be fixed at a certain value for several OAM modes generation under tunable refractive index conditions. The proposed fiber coupler is flexible and compact, making it a good candidate for tunable OAM generation and sensing systems.
We demonstrate a Sagnac interferometer (SI) based on a selective-filling photonic crystal fiber (SF-PCF), which is achieved by infiltrating a liquid with higher refractive index than background silica into two adjacent air holes of the innermost layer. The SF-PCF guides light by both index-guiding and bandgap-guiding. The modal birefringence of the SF-PCF is decidedly dependent on wavelength, and the modal group birefringence has zero value at a certain wavelength. We also theoretically and experimentally investigate in detail the transmission and temperature characteristics of the SI. Results reveal that the temperature sensitivity of the interference spectrum is also acutely dependent on wavelength and temperature, and an ultrahigh even theoretically infinite sensitivity can be achieved at a certain temperature by choosing proper fiber length. An ultrahigh sensitivity with -26.0 nm/°C (63,882 nm/RIU) at 50.0 °C is experimentally achieved.
A double-filled photonic crystal fiber (PCF) was fabricated by filling liquids of different indexes into two air holes in the cladding. The core mode coupled to the local cladding modes LP(01) and LP(11) in the 1310 and 1550 nm wavebands, respectively. Due to the unique characteristics of the mode coupling, the resonant peaks in different resonance areas shifted to the opposite directions with the variations of the temperature or the force. The double-filled PCFs achieved in this work showed useful applications in the simultaneous measurement of both the temperature and the force.
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